The present invention is directed to an electrode coil for a high intensity discharge (HID) lamp and to a method of making an electrode coil for a HID lamp.
As shown in FIG. 1, a conventional HID lamp includes a tube 10 with two electrode coils 12 therein that are typically placed at opposing ends of tube 10. Tube 10 is filled with an appropriate gas and fill material, and sealed. Each electrode coil 12 includes a tungsten shank 14 with a tungsten wire coil 16 adjacent to a free end of tungsten shank 14 inside tube 10.
Electrode coil 12 has been conventionally made with a coiled coil or by back winding tungsten wire to form a second layer of wire wound in a direction opposite to the winding direction of the first layer. These manufacturing methods have not proven entirely satisfactory.
The coiled coil method includes the steps of winding a primary tungsten wire around a primary tungsten mandrel and then winding the coiled wire and primary mandrel around a secondary molybdenum mandrel. The coiled coil is heat-treated, cut to length, and heat-treated again. The secondary molybdenum mandrel is dissolved in acid and replaced with a tungsten shank. An example of a coiled coil electrode coil is shown in FIG. 2.
The coiled coil method is generally cost effective because the manufacturing equipment is largely automated. However, the insertion of the tungsten shank can cause the primary tungsten mandrel to crack, which is a basis for rejecting the electrode coil.
The back winding method includes the step of winding a tungsten wire around a retractable steel pin. After a predetermined number of turns or distance, the winding direction is reversed (for example, from left-to-right to right-to-left) and the wire is wound back over itself to form a second layer. Subsequently, several turns of the first layer may be left exposed, the steel pin removed, the coil oriented properly, and the tungsten shank inserted. An example of a back wound electrode coil is shown in FIG. 3.
Although the back winding method produces fewer problems than the coiled coil method when the tungsten shank is inserted, the back wound coil does not hold its shape well. Moreover, the process is more labor intensive as the asymmetrical coil must be oriented properly on the tungsten shank. The orientation of the coil takes additional time and these machines quickly reach capacity limits.
U.S. Pat. No. 4,105,908 discloses a back wound coiled coil electrode. A coil wrapped around a primary mandrel is wrapped around a secondary mandrel and back wound over itself to form a two-layer coiled coil, such as shown in FIG. 4. However, manufacture of this electrode coil enjoys the problems of both the above-noted methods.
U.S. Pat. No. 2,523,033 is not related to the manufacture of electrode coils, but is of general interest because it discloses a double layer coil in a lamp. The lamp includes a filament that expands and contracts axially during use. A spring portion of the filament absorbs the stress of elongation and contraction. As shown in FIG. 5, an in-lead 18 for the spring is thicker than filament 20 and is connected to filament 20 by butt-welding 22 the ends of the small diameter filament 20 to the large diameter in-lead 18. A first layer of wire 24 is wound around filament 20. The wire 24 has a diameter equal to the difference between the radii of filament 20 and in-lead 18. A second layer of wire 26 is screwed onto first layer 24 and onto in-lead 18. The combination of first and second layers of wire 24 and 26 reinforces butt-weld 22 by absorbing some of the mechanical strain.